Disposable absorbent articles (such as diapers) include typically an absorbent core structure with superabsorbent polymers, typically hydrogel-forming water-swellable polymers (also referred to as absorbent gelling material, AGM, or superabsorbent polymers, SAP's). This polymer material ensures that in use, large amounts of bodily fluids, e.g. urine, can be absorbed by the article and locked away, thus providing low rewet and good skin dryness.
These water-swellable or superabsorbent polymers need to have adequately high sorption capacity, as well as adequately high gel strength. Sorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Together with other properties of the gel, gel strength relates to the tendency of the swollen polymer particles (i.e. gel) to resist deformation under an applied stress in the absorbent article. The gel strength needs to be high enough in the absorbent article so that the particles do not deform too much and thereby fill the capillary void spaces to an unacceptable degree, which would cause so-called gel blocking. This gel-blocking inhibits the rate of fluid uptake and/or the fluid distribution: i.e. once gel-blocking occurs, it can substantially impede the distribution of fluids to relatively dry zones or regions in the absorbent article; then, leakage from the absorbent article can take place well before the superabsorbent polymer particles are fully saturated or before the fluid can diffuse or wick past the “blocking” particles into the rest of the absorbent article. Thus, it is important that the superabsorbent polymers (when incorporated in an absorbent structure or article) maintain a high wet-porosity and have a high resistance against deformation thus yielding high permeability for fluid transport through the swollen gel bed.
Absorbent polymers with relatively high permeability can be made by increasing the level of internal crosslinking or surface crosslinking, which increases the resistance of the swollen gel against deformation by an external pressure (such as the pressure caused by the wearer), but these techniques typically also reduce the absorbent capacity of the gel undesirably.
In addition, there is also a need for superabsorbent polymer particles that have a greater speed of absorption. It has been found that the prior art superabsorbent polymers that may have high gel strength, may often not have a high absorption speed.
In recent years, some absorbent polymers that are cross-linked by nano-sized clay particles have been proposed. Unlike some superabsorbent material whereby clay is added after polymerization, it has been found to be important that the clay is added in nano-size prior to polymerization, to ensure the clay form strong crosslinks between the polymers. This is for example described in “Nanocomposite Polymer Gels”; Schexnailder/Schmidt; Coloid Polym Sci (2009) 287: 1-11. Some of said clay-crosslinked polymers form elastic or stretchable hydrogels upon swelling. For example, water-containing hydrogel shaped or molded articles, comprising certain specific isopropyl polyamides cross-linked by certain clay particles are described in Macromolecules 2002, 35, 10162-10171 (Kazutoshi Haraguchi et all); these elastic, shaped hydrogels are intended for medical purposes where they can be used in applications where they can de-water quickly, and thus shrink, upon demand, e.g. driven by temperature changes. WO09/041870 and WO2009/041903 describe the desire to make clay-linked polyacrylates, which provide a better absorbency, but that polyacrylates cannot be linked by nano-size clay particles successfully, because the clay agglomerates in the presence of acrylate or acrylic acid. They teach thereto fibers, foams and gels (that may be made in particles) of clay-linked hydrogels, made by mixing nano-size clay particles and acrylic esters in a liquid to form clay-linked polyacrylic esters, that may be shaped in foams, fibers, gels etc. These polyacrylic ester shapes are then hydrolyzed using conventional hydrolysis techniques in order to obtain polyacrylate shapes (e.g. foams, fibers, gels, etc.). However, the hydrolyses of complete foams, fibers or gels, or even batches of finished particles of polyacrylic esters is a very slow and energy-demanding process, because the penetration of the hydrolysis solution is driven by diffusion only which is a generally slow process, in particular if larger shapes such as foams or gels need to be hydrolyzed (internally).
Furthermore, hydrolysis of ground particles would cause the particles to form a gel blocks (the particles would stick together due to the hydrolysis solvent liquid), which would then need to be dried and grinded, sieved etc. to obtain particles. Thus, with the above described processes, this process would need to be done twice.
In addition, it is difficult to achieve a very homogeneous hydrolysis throughout the entire polymer particles, i.e. some parts of the polymer may be hydrolyzed earlier and to a larger extent than others. Furthermore, by-products from the hydrolysis (such as methanol or ethanol) would need to be removed from the product, and the level of these by-products would need to be brought to very low levels (toxicity, odour).
Thus, the proposed clay-crosslinked polyester gel blocks and foams, or even fibers or particles, and the hydrolysis thereof are not suitable for commercial scale production of clay-linked polyacrylates (particles). The present invention provides processes whereby said hydrolysis can be avoided, or if necessary, can be done in an effective manner.